EEP 37: Funs with names

This adds optional names to fun expressions. A named fun expression
is parsed as a tuple `{named_fun,Loc,Name,Clauses}` in erl_parse.

If a fun expression has a name, it must be present and be the same in
every of its clauses. The function name shadows the environment of the
expression shadowing the environment and it is shadowed by the
environment of the clauses' arguments. An unused function name triggers
a warning unless it is prefixed by _, just as every variable.
Variable _ is allowed as a function name.

It is not an error to put a named function in a record field default
value.

When transforming to Core Erlang, the named fun Fun is changed into
the following expression:

    letrec 'Fun'/Arity =
               fun (Args) ->
                       let <Fun> = 'Fun'/Arity
                       in Case
    in 'Fun'/Arity

where Args is the list of arguments of 'Fun'/Arity and Case the
Core Erlang expression corresponding to the clauses of Fun.

This transformation allows us to entirely skip any k_var to k_local
transformation in the fun's clauses bodies.

1 files changed, 122 insertions, 4 deletions

diff --git a/lib/syntax_tools/src/erl_syntax.erl b/lib/syntax_tools/src/erl_syntax.erl
index 409805e95f..5911502960 100644
--- a/lib/syntax_tools/src/erl_syntax.erl
+++ b/lib/syntax_tools/src/erl_syntax.erl
@@ -226,6 +226,10 @@
 	 module_qualifier/2,
 	 module_qualifier_argument/1,
 	 module_qualifier_body/1,
+	 named_fun_expr/2,
+	 named_fun_expr_arity/1,
+	 named_fun_expr_clauses/1,
+	 named_fun_expr_name/1,
 	 nil/0,
 	 operator/1,
 	 operator_literal/1,
@@ -442,28 +446,30 @@
 %%   <td>match_expr</td>
 %%   <td>module_qualifier</td>
 %%  </tr><tr>
+%%   <td>named_fun_expr</td>
 %%   <td>nil</td>
 %%   <td>operator</td>
 %%   <td>parentheses</td>
-%%   <td>prefix_expr</td>
 %%  </tr><tr>
+%%   <td>prefix_expr</td>
 %%   <td>receive_expr</td>
 %%   <td>record_access</td>
-%%  </tr><tr>
 %%   <td>record_expr</td>
+%%  </tr><tr>
 %%   <td>record_field</td>
 %%   <td>record_index_expr</td>
 %%   <td>rule</td>
-%%  </tr><tr>
 %%   <td>size_qualifier</td>
+%%  </tr><tr>
 %%   <td>string</td>
 %%   <td>text</td>
 %%   <td>try_expr</td>
-%%  </tr><tr>
 %%   <td>tuple</td>
+%%  </tr><tr>
 %%   <td>underscore</td>
 %%   <td>variable</td>
 %%   <td>warning_marker</td>
+%%   <td></td>
 %%  </tr>
 %% </table></center>
 %%
@@ -506,6 +512,7 @@
 %% @see macro/2
 %% @see match_expr/2
 %% @see module_qualifier/2
+%% @see named_fun_expr/1
 %% @see nil/0
 %% @see operator/1
 %% @see parentheses/1
@@ -554,6 +561,7 @@ type(Node) ->
 	{'catch', _, _} -> catch_expr;
 	{'cond', _, _} -> cond_expr;
 	{'fun', _, {clauses, _}} -> fun_expr;
+	{named_fun, _, _, _} -> named_fun_expr;
 	{'fun', _, {function, _, _}} -> implicit_fun;
 	{'fun', _, {function, _, _, _}} -> implicit_fun;
 	{'if', _, _} -> if_expr;
@@ -5616,6 +5624,110 @@ fun_expr_arity(Node) ->
 
 
 %% =====================================================================
+%% @doc Creates an abstract named fun-expression. If `Clauses' is
+%% `[C1, ..., Cn]', the result represents "<code>fun
+%% <em>Name</em> <em>C1</em>; ...; <em>Name</em> <em>Cn</em> end</code>".
+%% More exactly, if each `Ci' represents
+%% "<code>(<em>Pi1</em>, ..., <em>Pim</em>) <em>Gi</em> -> <em>Bi</em></code>",
+%% then the result represents
+%% "<code>fun <em>Name</em>(<em>P11</em>, ..., <em>P1m</em>) <em>G1</em> ->
+%% <em>B1</em>; ...; <em>Name</em>(<em>Pn1</em>, ..., <em>Pnm</em>)
+%% <em>Gn</em> -> <em>Bn</em> end</code>".
+%%
+%% @see named_fun_expr_name/1
+%% @see named_fun_expr_clauses/1
+%% @see named_fun_expr_arity/1
+
+-record(named_fun_expr, {name :: syntaxTree(), clauses :: [syntaxTree()]}).
+
+%% type(Node) = named_fun_expr
+%% data(Node) = #named_fun_expr{name :: Name, clauses :: Clauses}
+%%
+%%	Name = syntaxTree()
+%%	Clauses = [syntaxTree()]
+%%
+%%	(See `function' for notes; e.g. why the arity is not stored.)
+%%
+%% `erl_parse' representation:
+%%
+%% {named_fun, Pos, Name, Clauses}
+%%
+%%	Clauses = [Clause] \ []
+%%	Clause = {clause, ...}
+%%
+%%	See `clause' for documentation on `erl_parse' clauses.
+
+-spec named_fun_expr(syntaxTree(), [syntaxTree()]) -> syntaxTree().
+
+named_fun_expr(Name, Clauses) ->
+    tree(fun_expr, #named_fun_expr{name = Name, clauses = Clauses}).
+
+revert_named_fun_expr(Node) ->
+    Pos = get_pos(Node),
+    Name = named_fun_expr_name(Node),
+    Clauses = [revert_clause(C) || C <- named_fun_expr_clauses(Node)],
+    case type(Name) of
+	var ->
+	    {named_fun, Pos, concrete(Name), Clauses};
+	_ ->
+	    Node
+    end.
+
+
+%% =====================================================================
+%% @doc Returns the name subtree of a `named_fun_expr' node.
+%%
+%% @see named_fun_expr/2
+
+-spec named_fun_expr_name(syntaxTree()) -> syntaxTree().
+
+named_fun_expr_name(Node) ->
+    case unwrap(Node) of
+	{named_fun, Pos, Name, _} ->
+	    set_pos(atom(Name), Pos);
+	Node1 ->
+	    (data(Node1))#named_fun_expr.name
+    end.
+
+
+%% =====================================================================
+%% @doc Returns the list of clause subtrees of a `named_fun_expr' node.
+%%
+%% @see named_fun_expr/1
+
+-spec named_fun_expr_clauses(syntaxTree()) -> [syntaxTree()].
+
+named_fun_expr_clauses(Node) ->
+    case unwrap(Node) of
+	{named_fun, _, _, Clauses} ->
+	    Clauses;
+	Node1 ->
+	    (data(Node1))#named_fun_expr.clauses
+    end.
+
+
+%% =====================================================================
+%% @doc Returns the arity of a `named_fun_expr' node. The result is
+%% the number of parameter patterns in the first clause of the
+%% named fun-expression; subsequent clauses are ignored.
+%%
+%% An exception is thrown if `named_fun_expr_clauses(Node)'
+%% returns an empty list, or if the first element of that list is not a
+%% syntax tree `C' of type `clause' such that
+%% `clause_patterns(C)' is a nonempty list.
+%%
+%% @see named_fun_expr/1
+%% @see named_fun_expr_clauses/1
+%% @see clause/3
+%% @see clause_patterns/1
+
+-spec named_fun_expr_arity(syntaxTree()) -> arity().
+
+named_fun_expr_arity(Node) ->
+    length(clause_patterns(hd(named_fun_expr_clauses(Node)))).
+
+
+%% =====================================================================
 %% @doc Creates an abstract parenthesised expression. The result
 %% represents "<code>(<em>Body</em>)</code>", independently of the
 %% context.
@@ -5978,6 +6090,8 @@ revert_root(Node) ->
 	    revert_match_expr(Node);
 	module_qualifier ->
 	    revert_module_qualifier(Node);
+	named_fun_expr ->
+	    revert_named_fun_expr(Node);
 	nil ->
 	    revert_nil(Node);
 	parentheses ->
@@ -6219,6 +6333,9 @@ subtrees(T) ->
 		module_qualifier ->
 		    [[module_qualifier_argument(T)],
 		     [module_qualifier_body(T)]];
+		named_fun_expr ->
+			[[named_fun_expr_name(T)],
+			 named_fun_expr_clauses(T)];
 		parentheses ->
 		    [[parentheses_body(T)]];
 		prefix_expr ->
@@ -6349,6 +6466,7 @@ make_tree(list_comp, [[T], B]) -> list_comp(T, B);
 make_tree(macro, [[N]]) -> macro(N);
 make_tree(macro, [[N], A]) -> macro(N, A);
 make_tree(match_expr, [[P], [E]]) -> match_expr(P, E);
+make_tree(named_fun_expr, [[N], C]) -> named_fun_expr(N, C);
 make_tree(module_qualifier, [[M], [N]]) -> module_qualifier(M, N);
 make_tree(parentheses, [[E]]) -> parentheses(E);
 make_tree(prefix_expr, [[F], [A]]) -> prefix_expr(F, A);